Fast writing bistable storage tube and method of operation

ABSTRACT

A direct viewing bistable storage tube employing post deflection acceleration and method of operation is described in which the storage target includes a contacting collector mesh electrode and a target electrode provided on opposite sides of a phosphor storage dielectric. A more positive voltage is applied to the target electrode than to the collector electrode during writing. The result is an increase of the maximum stored writing speed of four to five times, apparently because the secondary electrons produced within the phosphor storage dielectric by the writing beam are attracted toward the target electrode and away from the bombarded surface of such phosphor layer so that the initial voltage of the charge image is of a more positive potential. This causes the initial charge image to be above the first crossover voltage even at such faster writing speeds which enables such charge image to be stored bistably by the charging action of the low velocity flood electrons. An intermediate layer of light transparent insulating material may be provided between the target electrode and the phosphor layer to increase the breakdown voltage of the dielectric and thereby enable such target electrode to be maintained at a higher positive voltage with respect to the collector electrode.

United States Patent 1191 Frankland et al. A

[ FAST WRITING BISTABLE STORAGE TUBE AND METHOD OF OPERATION [75] Inventors: Roger A. Frankland; Christopher J.

Curtin, both of Portland, Oreg.

[73] Assignee: Tektronix,Inc.,Beaverton,Oreg.

221 Filed: May 4,1970

211 Appl.No.: 34,072

Primary ExaminerCarl D. Quarforth Assistant Examiner-J. M. Potenza' Attorney-Stephen W. Blore, Joseph B, Sparkman and James Campbell, Jr. et al.

[ June 26, 1973 [57] ABSTRACT A direct viewing bistable storage tube employing post deflection acceleration and method of operation is described in which the storage target includes a contacting collector mesh electrode and a target electrode provided on opposite sides of a phosphor storage dielectric. A more positive voltage is applied to the target electrode than to the collector electrode during writing. The result is an increase of the maximum stored writing speed of four to five times, apparently because the secondary electrons produced within the phosphor storage dielectric by the writing beam are attracted toward the target electrode and away from the bombarded surface of such phosphor layer so that the initial voltage of the charge image is of a more positive potential. This causes the initial charge image to be above the first crossover voltage even at such faster writing speeds which enables such charge image to be stored bistably by the charging action of the low velocity flood electrons. An intermediate layer of light transparent insulating material may be provided between the target electrode and the phosphor layer to increase the breakdown voltage of the dielectric and thereby enable such target electrode to be maintained at a higher positive voltage with respect to the collector electrode.

14 Claims, 3 Drawing Figures PAIENIEDwuzs 1915 3. 742.286

FIG. 2

ROGER A. FRANKLAND CHRISTOPHER J. CURTIN INVENTORS.

BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS FAST WRITING BISTABLE STORAGE TUBE AND METHOD OF OPERATION BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to charge image storage apparatus and, in particular, to direct viewing bistable storage tubes and method of operation. While such storage tubes have many other uses, they are frequently employed as display devices for cathode ray oscilloscopes and computer terminals.

The present invention is an improvement on the simplified type of storage tubes shown in US. Pat. Nos. 3,214,631 and 3,293,473 of R. H. Anderson which employ storage targets including a storage dielectric in the form of a layer of phosphor material which also functions as the viewing screen and is coated on the glass faceplate of the tube over a light transparent target electrode. Unlike these earlier tubes, the storage tube of the present invention employs an additional collector electrode mesh in contact with the opposite side of the phosphor storage dielectric layer from the target electrode, and also employs a post deflection acceleration electrode positioned between the deflection plates and the flood gun cathodes. Other differences will be apparent from the following description of the present storage tube.

The storagetube of the present invention has several advantages over conventional direct viewing bistable storage tubes including a faster writing speed and a stored image display of high brightness. For example, writing speeds on the order of 125,000 centimeters per second and display brightness of 12 foot-lamberts have been achieved with bistable storage operation using a storage dielectric of P-l type phosphor particles without any secondary emissive additive. This represents a gain in writing speed of about four to five times that achieved by earlier storage tubes using a similar storage dielectric. In addition, conventional nonstored operation of thetube has been improved due, among other things, to the use of. post deflection acceleration to provide a nonstored image display of 300 foot-lamberts brightness at a maximum writing speed of 500 centimeters per microsecond. An extremely rugged and inexpensive storage tube is provided since the collector electrode mesh is in the form of a conductive coating supported onthe phosphor layer, and is not in the form of a separate wire mesh, so that it does not have the image resolution and target size limitation problems associated with storage tubes having a transmission type storage target and separate phosphor viewing screen.

While the exact nature of operation of the storage tube of the present invention is not clearly understood, the following theory of operation is believed to be correct and is offered for purposes of clarity, but the invention should not be limited by such theory. As a result of the post deflection acceleration through a high potential gradient electrical field, on the order of to 20 kilovolts, the writing beam penetrates the storage surface of the phosphor layer storage dielectric and causes secondary electrons to be produced within such layer leaving a charge image of positive voltage on such surface. The low velocity flood electrons cause bistable storage of this charge image by producing secondary electron emission at the surface of the phosphor layer, such secondary electrons being emitted from the layer and collected by the collector electrode. A more efflcient secondary emission bistable storage operation is achieved by applying a high positive voltage to the target electrode to enable the initial voltage of the charge image produced by the writing beam to be a more positive voltage. Thus, when the target electrode is provided with a more positive voltage than the collector electrode to produce a high positive potential gradient between the target electrode and the collector electrode, the secondary electrons produced by the writing beam within the storage dielectric layer are attracted toward the target electrode and away from the storage surface so that such secondary electrons cannot reduce the positive voltage of the initial charge image. At fast writing speeds in previous storage tubes, the secondary electrons of the writing beam were allowed to reach the storage surface and caused the potential of the initial charge image to be reduced below the first crossover voltage so that bistable storage by the uniform bombardment of the flood electrons is not possible. This problem was discovered and overcome by the present invention so that the storage tube of the present invention is capable of a faster maximum writing speed during its storage operation.

It is, therefore, one object of the present invention to provide an improved bistable storage apparatus and method of operation which is capable of a faster maximum writing speed when producing a stored charge ima e.

Another object of the invention is to provide a direct viewing bistable storage tube in which post deflection acceleration of the writing beam is employed along with a storage target having a light transparent target electrode and a collector mesh electrode positioned on opposite sides of a phosphor storage dielectric with the target electrode.

Still another object of the invention is to provide such a storage tube and method of operation in which a more positive voltage is applied to the target electrode than to the collector electrode to enable storage at a faster writing speed.

A further object of the present invention is to provide such a storage tube in which the post deflection acceleration electrode is positioned between the writing beam deflection means and the flood gun cathode.

An additional object of the present invention is to provide 'such a storage tube and method of operation in which the writing beam is caused to penetrate into the storage dielectric layer and produce secondary electrons within such layer which are attracted toward the target electrode away from the bombarded surface of the storage dielectric, at least until the low velocity flood electrons cause bistable storage of the charge image.

A still further object of the present invention is to provide such a storage tube in which an intermediate layer of light transparent insulating material having a higher dielectric constant than the phosphor material is provided between the phosphor layer and the target electrode to provide a higher voltage breakdown strength for the dielectric between the target electrode and the collector electrode.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawings of which:

FIG. 1 is a side elevation view of one embodiment of the storage tube of the present invention with parts broken away for clarity;

FIG. 2 is an end elevation view taken along the line 2-2 of FIG. 1; and

FIG. 3 is a vertical section view taken along the line 3--3 of FIG. 2 shown on an enlarged scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIGS. 1 and 2, a direct viewing bistable storage tube in accordance with the present invention includes an evacuated envelope containing an electron gun 12 having a cathode 14 connected to a source of negative D.C. voltage of about 3 kilovolts for producing a writing beam 16 of high velocity electrons. The writing beam is deflected to form the charge image by any suitable deflection means, such as a pair of vertical deflection plates 18 and a pair of horizontal deflection plates 20 mounted within the envelope 10, or by electromagnetic deflection coils (not shown) positioned outside of the envelope. The writing beam 16 forms a charge image on the surface of a storage target 22 supported at the opposite end of the envelope from cathode 14 on a flat glass faceplate 24 forming a portion of the envelope.

The storage target is shown in greater detail in FIG. 3 and includes a target electrode 26 formed by a light transparent conductive film of tin oxide or the like coated on the inner surface of the glass faceplate 24. A storage dielectric layer 28 of phosphor material is provided on the faceplate over such target electrode. The storage dielectric 28 may contain any suitable phosphor material, such as manganese activated zinc orthosilicate, which is designated Zn SiO :MN and known as P-l phosphor. However, the storage dielectric may also contain a small percentage of secondary emissive material, such as magnesium oxide, in the form of small particles bonded to the surface of the phosphor particles in order to further increase the writing speed of such storage dielectric with a slight reduction in display brightness. A collector mesh electrode 30 is provided in contact with the bombarded surface of the phosphor storage dielectric 28 and may be in the form of a coating of metal providing a mesh of 200 lines per inch having a 75 percent electron transparency.

The collector electrode 30 is electrically connected to the exterior of the envelope 10 through a lead portion 32 of a layer of chrome plating or other suitable metal deposited on the surface of the glass faceplate and insulatingly spaced from the target electrode 26 by a gap 33. The glass faceplate 24 is sealed to a ceramic funnel portion 34 of the envelope 10 by means of a seal 36 of fused glass frit which may be a devitrifled glass that crystallizes during heating. Both the target electrode layer 26 and the lead portion 32 of the collector electrode 30 extend through the seal between the faceplate 32 and the sealing material 36 to the exterior of the envelope. Sources of D.C. supply voltages outside of the envelope are connected to the electrodes 26 and 30 so that, at least during the formation of a charge image by the writing beam, a more positive voltage is applied to the target electrode than to the collector electrode to provide a positive potential difference of about +300 volts between these electrodes to increase the writing speed of the storage tube, as discussed elsewhere herein.

In some cases, it may be desirable to increase the breakdown voltage strength of the dielectric material provided between the collector electrode 30 and the target electrode 26 to enable the above-mentioned high potential difference between such electrodes. For this reason, an intermediate layer 42 of light transparent insulating material, such as silicon dioxide or other fused glass material, having a higher dielectric constant than the phosphor material of the storage dielectric layer, is provided between the target electrode 26 and the phosphor layer 28. For example, an intermediate layer 42 of silicon dioxide, having a thickness of about 0.7 microns, has been employed with a phosphor layer 28 of P-] phosphor, having a thickness of about 25 microns, to enable a potential gradient of about +300 volts across such dielectric layers.

After the writing beam 16 forms a charge image on the phosphor storage dielectric layer 28, such charge image is stored as a bistable charge image in a conventional manner by the bombardment of such phosphor layer substantially uniformly with low velocity flood electrons emitted by a plurality of flood guns 44. Each of the flood guns includes a flood gun cathode 46 which may be connected to a D.C. potential of about +15 kilovolts. During storage, the collector electrode 30 is connected to a positive D.C. potential of +l5,l volts or about volts positive with respect to the flood gun cathode, while the target electrode 26 is connected to a positive D.C. potential of +l5,475 volts or about +475 volts positive with respect to the flood gun cathode.

A post deflection acceleration electrode 48 is provided between the flood gun cathodes 46 and the output ends of the horizontal deflection plates 20, and such acceleration electrode is connected to a D.C. voltage of about +15 kilovolts. A mesh electrode 50 connected to a D.C. voltage of about zero volts is positioned closely adjacent the output ends of the horizontal deflection plate and spaced from such acceleration electrode. The mesh electrode 50 is near the average voltage of the horizontal deflection plates and functions as an electrostatic shield which prevents the high voltage field of the acceleration electrode 48 from penetrating into the space between the horizontal deflection plates 20 and distorting the horizontal deflection field.

Since the writing gun cathode 14 is connected to a negative D.C. voltage of 3 kilovolts, such beam is only accelerated through a potential gradient of about 3 kilovolts before being deflected by the vertical deflection plates 18 and the horizontal deflection plates 20. The deflected beam is then accelerated through a high potential gradient of about 15 kilovolts due to the field provided between the mesh electrode 50 and the acceleration electrode 48 so that such beam strikes the storage dielectric layer with a total energy of about 18 kilovolts. This post deflection acceleration causes the writing beam to produce a light image of high brightness, up to 300 foot-lamberts, on the phosphor screen layer 28 and a writing speed of about 500 centimeters per microsecond for conventional nonstorage operation of the tube. During nonstored operation of the tube, the flood guns 44 may be cut off to prevent bistable storage. The post deflection acceleration of the writing beam also enables a faster writing speed during bistable storage operation of the tube when the target electrode 26 is at a highly positive voltage with respect to the collector electrode 30. As a result, the stored writing speed is increased four to five times, for example from 25,000 centimeters per second to about 125,000 centimeters per second for a storage dielectric layer consisting entirely of P-l phosphor with no secondary emissive additive. The light image display corresponding to the stored charge image has a brightness of about 12 foot-lamberts.

While the reason for the increased maximum stored writing speed is not clearly understood, it is believed that the post deflection acceleration of the writing beam causes such writing beam to penetrate through the bombarded storage surface of the phosphor layer 28 and to produce secondary electrons within the phosphor layer. These writing beam secondary electrons are attracted toward the target electrode 26 and away from such storage surface rather than toward the collector electrode 30 due to the more positive potential of such target electrode. This results in an initial charge image of more positive potential being produced on the bombarded surface of the phosphor layer so that even for faster writing speeds such charge image voltage is still above the first crossover voltage on the secondary electron emission curve of the storage dielectric so that the low velocity flood electrons can cause bistable storage of such charge image in a conventional manner. Thus, the low velocity flood electrons emitted by the flood gun cathode 46 bombard the surface of the phosphor layer 28 and cause secondary electrons to be emitted therefrom which are collected by the collector electrode 30. This causes the potential of the charge image to be driven up to an upper stable voltage corresponding to the collector electrode voltage, and causes the potential of the unwritten background target areas to be driven down to a lower stable voltage corresponding to the flood gun cathode voltage.

The intermediate insulating layer 42 blocks any D.C. field and prevents the secondary electrons produced within the phosphor layer by the writing beam from actually reaching the target electrode, but this does not matter since bistable storage of the charge image is accomplished by the flood electron within about 1 microsecond after writing. Thus, the AC. field produced across the intermediate insulating layer 42 between the writing beam and the target electrode is enough to attract secondary electrons of the writing beam away from the bombarded surface of the phosphor layer for a sufficient time to enable bistable storage.

The direct viewing bistable storage tube of the present invention also includes collimating electrodes 52 and 54 of conducting material coated on the inner surface of the envelope funnel 34 and connected, respectively, to positive D.C. voltages of 15,090 volts and 15,080 volts. These collimating electrodes cause the flood electrons to strike the phosphor storage dielectric substantially at right angles thereto and to uniformly distribute such flood electrons over the surface of such dielectric in a conventional manner. Also, the input signal applied to the vertical deflection plates 18 is supplied in a conventional manner through a vertical amplifier 56 having its input connected as the input terminal 58 of the cathode ray oscilloscope. The input terminal 58 may also be connected to a trigger generator circuit 60 which, upon receipt of a vertical input signal, generates a trigger pulse and applies such pulse to a sweep generator circuit 62 causing it to produce a ramp voltage sweep signal which is applied to the horizontal deflection plates 20.

In addition, a conventional graticule scale 64 of intersecting lines of light reflecting material may be provided on an exterior graticule plate 66 positioned over the exterior surface of the faceplate 24. Alternately, the graticule lines 64 may be provided on the inner surface of the faceplate and illuminated by light transmitted through the light transparent edge of such faceplate.

It will be obvious to those having ordinary skill in the art that many changes may be made in the abovedescribed preferred embodiment of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. A bistable charge image storage tube apparatus of fast writing speed in which the improvement comprises:

writing means for producing a writing beam of high velocity electrons and including deflection means for deflecting said beam to produce the charge image;

post deflection acceleration means for accelerating the writing beam through a high electrical field after said beam is deflected;

storage target means including a storage dielectric provided on a support member in position to be bombarded by said writing beam for producing a charge image on said storage dielectric by secondary electron emission, a target electrode provided on said support member beneath said storage dielectric, and a secondary electron collector electrode of mesh shape supported over the bombarded surface of the storage dielectric on the opposite side of said storage dielectric from the target electrode;

holding means for bombarding the storage target with low velocity electrons to cause bistable storage of said charge image by secondary electron emission; and

voltage means for applying a voltage to the target electrode which is more positive than the voltage on the collector electrode to produce a positive potential gradient across the storage dielectric at least during the formation of the stored charge image, said target and collector electrodes both being maintained at a more positive voltage than the cathode emitting the low velocity electrons of the holding means at least during storage.

2. Storage tube apparatus in accordance with claim 1 in which the collector electrode contacts the bombarded surface'of the storage dielectric.

3. Storage tube apparatus in accordance with claim 2 inwhich the storage dielectric is a layer of phosphor material and the support member is of light transparent insulating material, while the target electrode is of light transparent conductive material.

4. Storage tube apparatus in accordance with claim 3 in which the support member is a nonporous glass member.

5. Storage tube apparatus in accordance with claim 3 in which the storage target means includes an intermediate layer of light transparent insulating material, having a higher dielectric constant than said phosphor material, provided between the phosphor layer and the target electrode.

6. Storage tube apparatus in accordance with claim in which the intermediate layer is of fused glass material.

7. Storage tube apparatus in accordance with claim 2 in which the acceleration means includes an acceleration electrode having at least a portion thereof positioned between the deflection means and the cathode emitting the low velocity electrons of the holding means.

8. Storage tube apparatus in accordance with claim 7 in which the acceleration electrode is provided on the inner surface of the envelope of said tube.

9. Storage tube apparatus in accordance with claim 8 in which the acceleration electrode is a ring shaped coating of conducting material.

10. Storage tube apparatus in accordance with claim 9 which also includes a mesh electrode between the acceleration electrode and the output ends of the deflection plates providing said deflection means.

11. A method of operating a bistable charge image storage tube having a storage target including a storage dielectric supported over a target electrode and having a collector electrode positioned on the opposite side of the storage dielectric from the target electrode in which the improvement comprises:

detecting a writing beam of high velocity electrons to form a charge image on the bombarded surface of the storage dielectric;

accelerating the writing beam through a high electrical field of several thousand volts after said deflection and before said beam strikes the storage dielectric to cause said beam to penetrate into said surface of the storage dielectric and produce a plurality of first secondary electrons within said storage dielectric;

applying a more positive DC. voltage to said target electrode than to said collector electrode to attract said first secondary electrons toward the target electrode and away from said surface of the storage dielectric to provide the charge image with a more positive potential;

bombarding said surface of the storage dielectric substantially uniformly with low velocity electrons to cause other secondary electrons to be emitted by the storage dielectric to cause bistable storage of said charge image; and

collecting said other secondary electrons with said collector electrode.

12. A method of operation in accordance with claim 11 in which the acceleration of the writing beam is done in an acceleration region free of the low velocity electrons.

13. A method of operation in accordance with claim 12 in which said acceleration region is positioned between the source of said low velocity electrons and the deflection region of the writing beam.

14. A method of operation in accordance with claim 11 in which the collector electrode contacts the bombarded surface of the storage dielectric and the attraction of the first secondary electrons away from such surface is accomplished by applying a more positive voltage to the target electrode than to the collector electrode. 

2. Storage tube apparatus in accordance with claim 1 in which the collector electrode contacts the bombarded surface of the storage dielectric.
 3. Storage tube apparatus in accordance with claim 2 in which the storage dielectric is a layer of phosphor material and the support member is of light transparent insulating material, while the target electrode is of light transparent conductive material.
 4. Storage tube apparatus in accordance with claim 3 in which the support member is a nonporous glass member.
 5. Storage tube apparatus in accordance with claim 3 in which the storage target means includes an intermediate layer of light transparent insulating material, having a higher dielectric constant than said phosphor material, provided between the phosphor layer and the target electrode.
 6. Storage tube apparatus in accordance with claim 5 in which the intermediate layer is of fused glass material.
 7. Storage tube apparatus in accordance with claim 2 in which the acceleration means includes an acceleration electrode having at least a portion thereof positioned between the deflection means and the cathode emitting the low velocity electrons of the holding means.
 8. Storage tube apparatus in accordance with claim 7 in which the acceleration electrode is provided on the inner surface of the envelope of said tube.
 9. Storage tube apparatus in accordance with claim 8 in which the acceleration electrode is a ring shaped coating of conducting material.
 10. Storage tube apparatus in accordance with claim 9 which also includes a mesh electrode between the acceleration electrode and the output ends of the deflection plates providing said deflection means.
 11. A method of operating a bistable charge image storage tube having a storage target including a storage dielectric supported over a target electrode and having a collector electrode positioned on the opposite side of the storage dielectric from the target electrode in which the improvement comprises: detecting a writing beam of high velocity electrons to form a charge image on the bombarded surface of the storage dielectric; accelerating the writing beam through a high electrical field of several thousand volts after said deflection and before said beam strikes the storage dielectric to cause said beam to penetrate into said surface of the storage dielectric and produce a plurality of first secondary electrons within said storage dielectric; applying a more positIve D.C. voltage to said target electrode than to said collector electrode to attract said first secondary electrons toward the target electrode and away from said surface of the storage dielectric to provide the charge image with a more positive potential; bombarding said surface of the storage dielectric substantially uniformly with low velocity electrons to cause other secondary electrons to be emitted by the storage dielectric to cause bistable storage of said charge image; and collecting said other secondary electrons with said collector electrode.
 12. A method of operation in accordance with claim 11 in which the acceleration of the writing beam is done in an acceleration region free of the low velocity electrons.
 13. A method of operation in accordance with claim 12 in which said acceleration region is positioned between the source of said low velocity electrons and the deflection region of the writing beam.
 14. A method of operation in accordance with claim 11 in which the collector electrode contacts the bombarded surface of the storage dielectric and the attraction of the first secondary electrons away from such surface is accomplished by applying a more positive voltage to the target electrode than to the collector electrode. 